Ceres' Large Craters Have Been Obliterated Over Time

Where are all of Ceres' large craters? This is a question that has plagued astronomers for years, and now we might finally have the answer: Ceres used to have large craters, but they've been disappeared over time, likely as a result of the dwarf planet's composition.

Ceres has been around for 4.5 billion years, which means it should have been hit by many, many asteroids over the course of its lifetime. There are many small craters peppered on the surface, but even so, there aren't nearly as many as they should be. In a new study, published in Nature, researchers modeled the number of large craters that should have appeared on Ceres' surface from asteroid impacts, and found that the dwarf planet should have at least 40 craters that are more than 100 km in diameter, and 10-15 craters that measure 400 km across or more. But in reality, Ceres has only 16 craters that are 100 km wide, and none that are above 400 km (the largest is a mere 280 km across).

"We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres' peculiar composition and internal evolution," lead investigator Simone Marchi said in a NASA statement.

One theory for the missing craters involves Ceres forming in the outskirts of our Solar System, and then migrating to its current position. But according to the researchers, even if it migrated when the asteroid belt was less dense, it should still have many more impact craters than it has now. As a result, something must be making them disappear.

"Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years," Marchi said.

The researchers couldn't conclude the actual process of this obliteration with any certainty, but there are a few likely options. First, if there is a high concentration of a lower-density material, such as ice, underneath the high-density rock of the top layer of the surface, then the difference in density could cause the top layer to "smooth" itself out. There's also a possibility that Ceres has a past of frequent cryovolcanic activity, the eruptions from which could have covered up large craters from its ancient history and left a smoother surface for small impacts to occur.